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737NG - VNAV logic under speed intervention

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737NG - VNAV logic under speed intervention

Old 24th May 2020, 05:15
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Join Date: May 2020
Location: Xiamen
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Smile 737NG - VNAV logic under speed intervention

Hi guys, I was reading the FCOM and found something confusing.
The manual says:
If engaging speed intervention:
- During a path descent with flaps up on an idle leg, VNAV switches to VNAV SPD
- With flaps down, VNAV remains in VNAV PTH
- When a geometric path leg is active, VNAV remains in VNAV PTH
- While a vertical angle leg (GP x.xx on RTE LEGS page) is active, VNAV remains in VNAV PTH
What I wonder is, if speed intervention has been engaged prior to lower flaps, enter a geometric path leg or a vertical angle leg, which means the VNAV SPD has been annunciated, will it turn back to VNAV PTH(with speed intervention still engaged) when we put flaps down, or enter a geometric path leg or a vertical angle leg?
The other question is, what if we select SPEED from CDU to activate VNAV SPD instead of engaging speed intervention, will it turn back to VNAV PTH once encounter the criteria mentioned above?

Looking forward for answers, thanks a lot.
frankx is offline  
Old 5th Sep 2020, 20:30
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Join Date: May 2001
Location: Europa
Age: 55
Posts: 26
Hello Frankx,

I am assuming you are on approach when all this happens. The speed window opens so you can select your required speed (mind the speed VS required flapsetting).
You have to realize which MCP mode is active when you select speed Intervention. VNAV is still the active MCP mode. On your FMA’s you will see VNAV PATH in the pitch mode and possibly ARM or FMC SPEED in the A/T FMA. VNAV PATH will remain active when your FMC is in the approach mode, which is an internal mode. Your calculated descent Path would be maintained and speed reduced, but only if there is enough room for that.
That’s not what your asking, but important to realize the working of VNAV during an approach.

You probably mean to ask you have selected Speed Intervention to adjust your speed ( other than the FMC would have done) and then you might end up too high with respect to your FMC calculated Path it would revert to VNAV SPD. You would have to select a higher speed in the Speed Window to capture the FMC Path again and yes it would then recapture the path and indicate VNAV PATH on the pitch FMA.

I found a YouTube channel which explains some of the FMA stuff. Flighttrainer Aviation Apps or something alike

hope this helps.

regards,

N.N.C

N.N.C is offline  

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